Posted by: @uk_pete_2000

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With our house it takes 12hrs to lost 1° but only takes about 6-8hrs to heat it up. So our schedule is heat from 08.00hrs to 16.00hrs then off for the rest of the time. All I do is change the length of the running time to match the conditions outside. At present the house is gaining about 0.1 to 0.2° each day, but next week it might lose that as it gets colder.

At present we can run the house on either WC or flow temp, but it only needs to run at 25-27° flow, to get the house to 20°. Next year I'm hoping we can get it down to 23° and still maintain the same degree of comfort.

With a very low loss house running at very low flow temperatures its true that the reduction in efficiency from running at the only slightly higher flow temp required for part time heating may be outweighed by other factors (particularly if you have multiple pumps) thus making part time heating cost effective.  I can see that your house may well fall into this category.

In the more common case its nevertheless worth bearing in mind that the house continues to lose heat even when the heat pump is off, and you have to replace that heat when its switched back on.  In your example, where the house takes 12 hrs to fall 1C and 6hrs to heat up, the heat loss from the house to the outside air is only 5% less than if you were to keep it at a constant temperature 24*7.  You have to replace this heat loss, so you will need to supply 95% of the energy (relative to the full time heating case) in half the time, which means that a detlaT somewhere has to be double or more. 

For radiators its the deltaT between radiator and room, and actually it will have to be 2.3 times as much (because radiator emissions scale with deltaT^1.3).  In this case the reduction in COP will almost certainly wipe out the 5% saving in energy lost by the house, even allowing for fixed loads. 

With UFH its not quite clear what deltaT has to double to deliver 95% of the energy in half the time.  Its probably the deltaT between pipes in the UFH and the screed, which is small.  Furthermore at the low FTs typical of UFH systems, the variation in COP with FT is less severe.  Thus other factors may dominate.  Furthermore if, in this case, you can heat only when electricity is low cost, you will be well cheaper.

@jamespa thank you for that, I never understand the full maths behind it all and things like that. I just did it on my findings.

We did try on a couple of days, at the start of Jan and found running at 23° flow for 24hrs kept the house at the right temp all the time, but the actual running cost where 3 times normal.

In the end I just extended the run time to 12hrs on, 12hrs off and that kept the temp to within 1° loss.

@iraknic we just discussed you buffer tank “design” on our latest podcast recording (it will be out this Wednesday) and @heacol was actually ‘impressed’ that you were even able to get a DT of 1 with the way it’s been piped in.

@editor 🤣 thanks. Please provide is with a link for the podcast when its out.

Posted by: @editor

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@iraknic we just discussed you buffer tank “design” on our latest podcast recording (it will be out this Wednesday) and @heacol was actually ‘impressed’ that you were even able to get a DT of 1 with the way it’s been piped in.

I’m assuming that this means that as installer mistakes go this way of piping up a buffer is pretty bad? The correction should be to re-pipe it as a volumiser?

@judith the correct is to repipe as a volumiser or correctly as a 4-port buffer, with proper balancing on both sides of the hydraulic separation to minimise distortion (aka, blending).

@editor I must learn how to select an emoji to indicate extreme irony, or probably better still don’t indulge it in writing.
You kindly showed the right way to pipe up a 4 port buffer in post 2 for @iraknic but I don’t believe we have enough information to know whether it is needs to be piped as a volumiser instead. Unless I missed details of the whole house design earlier such any difference between flow temperature in the 2 zones. The flow temperatures given earlier are much too high for good efficiency.

@judith, I completely missed the irony. My bad. An irony emoji would have been perfect here, but I can’t think of one either! 😂

After recording with our guests today, it’s clear that the volumiser (if additional volume is even needed) is by far the most sensible solution. The complexity of balancing flows pre- and post-buffer is huge, and it’s easy to see why so few systems with buffers work efficiently.

Posted by: @editor

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it’s easy to see why so few systems with buffers work efficiently.

Erm...is it? 🤔 

I'm finding this discussion very difficult to follow without diagrams.

Even tho' I can broadly understand the text when reading it multiple times,
I'm losing track of who is advocating what changes.

@transparent I lack the skills as an animator to illustrate stratification and how it gets affected by distortion. I'm hoping we explained it better in our podcast video.

I had assumed this subject could be described with static diagrams,
which I could create if members here posted a doodle - literally on the back of an envelope...

If we need animation,
then we ought to find someone who can create this using ChatGPT.

Just dug this up with google

https://www.youtube.com/@ThermalStratifier

A properly designed buffer vessel shouldn't be impossible to achieve.

Bob